1. Field of the Invention
The field of the present invention is related to liquid-feed flame spray pyrolysis production of ultrafine and nano mixed-metal oxide particles and coatings thereof, and selected metalloorganic precursors and solvent systems for this process. By suitable combination of precursors, unusual phases can be produced that offer opportunities to serve as novel catalysts, photonic materials, sensing materials, fuel cell materials, conducting materials, transparent ceramics, etc.
2. Background Art
The literature describes numerous methods of preparing ultrafine and nanosized metal oxide powders and metal oxide coatings using chemical compounds as precursors. In general, these methods can be broadly classified as liquid or gas phase processing. Liquid phase approaches include sol-gel, precipitation, hydrothermal, and electrochemical processing. Gas phase approaches include spray pyrolysis, metal evaporation/oxidation, plasma spray, flame spray pyrolysis, laser ablation, ion implantation, physical vapor deposition (“PVD”), or chemical vapor deposition (“CVD”) methods. Some methods combine processes, e.g. spray-freeze drying.
Liquid phase approaches usually require large volumes of liquid that must be removed/eliminated during coating, or to produce unaggregated nanoparticles. In the latter case, the solvent must then be laboriously separated from the nanoparticles. Both situations lead to production limitations and commonly to non-crystalline products that must then be calcined to crystallize the desired phase, and then sintered if dense coatings are desired. Thus, these processes tend to be multistep, require solvent recycle and introduce waste disposal issues that make them energy, equipment and time intensive. Hence they are less practical than gas phase methods.
Of the gas phase processing methods, CVD, spray pyrolysis, plasma spray, laser ablation, and flame spray pyrolysis (“FSP”), are used commercially to make ultrafine (100–500 nm average particle size “APS”) and nano (<100 nm APS) metal oxide powders and coatings.
Metal evaporation methods can be used to form metal coatings or powders that can be subsequently oxidized to metal oxides. Metal evaporation powder processing provides a variety of high purity, single-metal oxide nanopowders but offers limited opportunity for mixed-metal oxide powder or coating production because miscible metal alloy precursors are often difficult or impossible to produce.
Spray pyrolysis has been used to produce many types of single-metal oxide powders and some mixed-metal oxide powders and coatings. The process involves aerosolization of precursors dissolved in a wide variety of solvents. The initially formed aerosol droplets lose solvent by evaporation in a hot-walled reactor, and depending on the residence time in the reactor and the thermal environment, spherical powders are produced that are either hollow, porous and amorphous, or solids. Both ultrafine and nanosized powders can be formed, however the need for a hot-walled reactor often leads to particle aggregation unless very low number densities are used, limiting the potential for large scale production. Coatings made by this process with plasma assists are known, but the process has not been generally adopted except for making thermal barrier coatings. Furthermore, the literature reports that efforts to produce mixed-metal oxide powders rarely result in compositionally homogeneous materials with controlled phase or if successful, do not provide nanopowders. The products are often amorphous materials that require further heat treatment to obtain the desired phase. These same negative attributes make producing coatings very difficult.
PVD and laser ablation methods use high-energy electron or laser beams to ablate materials from targets of the composition desired in the coating or powders to be formed. These processes often require controlled atmospheres and because of the high energies involved are energy and equipment intensive. Furthermore, the targets themselves can require special processing to achieve compositions that give the correct stoichiometry in the coatings or powders produced which may not be the initial composition of the target.
Chemical vapor deposition takes many forms. The basic concept is one in which a volatile organometallic or metalloorganic compound is evaporated by heating, and entrained in a gas flow, or by vacuum sublimation. The material is then decomposed using a variety of processes that can involve heat, plasma, light or a combinations of these to produce powders in the gas phase or coatings on substrates. For coatings on substrates, the materials must decompose at, or near the substrate and then adhere which typically means that the substrate must be heated using a second heat source. Efforts to produce mixed-metal oxides are known and successful but again control of stoichiometry and phase are difficult because mixing in the gas phase, and simultaneous and uniform decomposition is frequently very difficult to obtain. Furthermore, the processes almost always involve closed wall reactors where the total pressures as well as the pressures and concentrations of the individual precursors must be closely controlled.
FSP is the primary commercial method of making kiloton quantities of ultrafine (titania for the paint industry) and nanopowders (fumed silica and carbon black). Carbon black is produced via numerous processes in a reducing flame. Ultrafine and nano-TiO2 and SiO2 (fumed) are produced from volatile TiCl4 or SiCl4 in H2/O2 combustion flames. The process involves flame hydrolysis. Combustion (700–1500° C.) generates nanopowders as ceramic soot, and chlorine and HCl as byproducts. Although, these byproducts are easily removed. they are toxic, corrosive pollutants, as are the starting metal chlorides. Thus, the commercial processes are highly equipment and energy intensive simply because of the pollution and corrosion control equipment. The resulting powders (e.g. titania) are identical to those produced by metal evaporation, or FSP of metal alkoxides but offer lower cost because the process is mature and scaled, although the products are frequently contaminated by chlorine.
Despite these drawbacks, the formation of coatings and monoliths using FSP is well-known in the optical and photonics fields, especially for the manufacture of boules for pulling fibers for optical cables. The process was first reported in the early '70s but has been used to produce a variety of coatings. Flame spray coating generates nanoparticles for example of silica and/or germania under conditions where they stick to the surface of a perform, and thereafter sinter to the surface to form a defect free, dense and high purity coating. The process can be repeated sufficient numbers of times that the coatings eventually become thick enough to form monolithic materials that can then be used as boules for pulling optical fibers. The thickness of each coating layer is determined only by the number of traverses of the torch across the mandrel/substrate.
The coating material can be varied so that it has a graded index of refraction. For example, mixtures of SiCl4 and GeCl4 can be varied in the process to create slight decreases in refractive index at the exterior allowing graded indices of refraction in the fibers following pulling. This approach is three decades old. More recently, Skandan et al. and Choy et al. have independently reported making coatings with controlled porosity using a nearly identical approach in which the deposited particles are selectively sintered to the substrate to control pore size and density.
Unfortunately, as noted in review articles on metal chloride FSP the production of mixed-metal oxides is relatively undeveloped, disadvantages of which include difficulties in producing multicomponent materials, low production rates, and hazardous gaseous reactants and byproducts, in addition to formation of hard agglomerates in the gas phase, leading to difficulties in producing high-quality bulk materials.
Doped mixed-metal nanopowders have been prepared in counterflow H2/O2 diffusion flames employing chlorides or oxychlorides of Ti, Al, Si, Ge, and V. However, mullite powders prepared from AlCl3 and SiCl4 gave rise to particles of individual metal oxides or of a metal oxide coated with a second metal oxide. Laminar diffusion FSP with H2/O2 has been used to prepare particles which are solid solutions at low Si content, but have a silica coating at higher Si content. Aside from issues of toxicity, corrosion, and pollution, the metal chlorides must be quite volatile for successful FSP production of metal oxide particles, and thus this method is not suitable as a general method for mixed-metal oxide powders.
Hunt et al., in U.S. Pat. Nos. 5,652,021; 5,858,465; 5,863,604; and 6,013,318, describe the use of reagents which vaporize when the liquid organic solution is burned and the resulting vapors are deposited onto a substrate that is positioned so that it is in or just beyond the flame's end. However, Hunt et al.'s description of the process suggests that the reagent is not changed during the combustion process, but rather that homogeneous nucleation in the gas phase is avoided and adherence is obtained by flame heating of the substrate, i.e. the process is similar in this respect to CVD. The process can be run in an open atmosphere, and the coatings can form epitaxially and are dense. Examples using two different solvents, ethanol and toluene, are given and using two different precursors, metal acetylacetonates and 2-ethylhexanoates. However, in making yttria-stabilized zirconia (YSZ), toluene solutions of the 2-ethylhexanoate were required to produce good coatings.